Synthetic vascular prostheses have been widely utilized in clinical medicine as conduit replacements and bypasses for large and small vessels in patients with inadequate autogenous replacement vein due to prior chemotherapy, previous harvest, phlebitis or other vascular pathologies. The use of synthetic vascular prostheses, particularly for microvascular reconstruction, coronary and below knee bypass grafting, has not been entirely satisfactory, however, due to the high rate of short-term thrombosis and long-term intimal hyperplasia associated with their use. Therefore, the need exists for synthetic vascular prostheses whose biocompatibility and performance approaches that of autogenous vein.
While a number of techniques have been employed to improve the patency of synthetic vascular prostheses, including the topical and systemic application of antiplatelet agents and the use of arteriovenous fistulas, satisfactory results have yet to be achieved. The failure of these techniques to attain prolonged patency in synthetic grafts is due no doubt to their inability to remedy or even alleviate one of the primary causes of prosthesis associated thrombogenicity, the presence of trapped gas on the synthetic surface and the resulting formation upon implant of a significant area of blood-gas interface.
Although the mechanism of prosthesis associated thrombosis is not completely understood, one major factor implicated in triggering thrombogenesis on biomaterials, and one overlooked by most researchers in the field, is the presence of small pockets of undissolved gas trapped in or on surfaces, crevices, fissures, and other asperities and within the structural matrices of synthetic materials. These gas pockets, which may be macroscopic or microscopic in size, have been termed gas nuclei. Scanning electron micrography of vascular prostheses has demonstrated that disproportionate platelet adhesion generally occurs at the site of nuclei entrapment.
Early studies with bubble and rotating disc oxygenators have shown that direct contact with gaseous oxygen is damaging to various blood components. For instance, the formation of blood-gas interfaces cause plasma protein denaturation, which in turn leads to sludging, clotting factor activation, alteration of immune system functions, activation of the complement system and the production of lipid emboli. From these and related observations it became clear that the formation of blood-gas interfaces on implanted biomaterials, due to the direct contact of blood with undissolved gas nuclei, could lead to thrombosis, and result in a significant decrease in the patency of the biomaterial.
Gas nuclei, which contribute to the formation of a blood-gas interface, have since been shown to cause thrombus formation on many synthetic biomaterials. Thrombi resulting from trapped gas nuclei are especially prevalent and problematic in biomaterials such as expanded polytetrafluoroethylene (PTFE), which are commonly used in small caliber prostheses. PTFE is, in fact, one of the most widely used of the microvascular graft materials. By way of example, the high rate of thrombosis associated with the use of PTFE may be best explained by its hydrophobicity and its microporous or fibril structure in which a large gas volume is trapped. These are both factors which have been found to contribute significantly to the presence and stability of gas nuclei.
The ability of a surface, particularly a porous surface, to trap gas once it is immersed in a liquid is directly related to the material's hydrophobicity and to the nature of the surrounding liquid. For instance, a non-polar liquid can enter a pore or crevice in a hydrophobic material more easily than an aqueous liquid can and thereby displace any gas which may be trapped within. Hydrophobic, microporous PTFE contains about 70% gas by volume and has an 80% internal gaseous surface area. Immersion of PTFE in water for one month displaces only 10% of the undissolved gas trapped on the material's surface and within its structural matrix. PTFE immersion in a non-polar compound such as acetone, however, displaces 95-97% of the trapped gas within 24 hours.
Numerous methods have been employed with varying degrees of success to extract trapped gas from biomaterials. These processes are generally known as priming techniques. When these methods are successful in ridding the material of gas nuclei, the term "denucleation" has been coined and may be more appropriate.
The basic objective of denucleation is the replacement of the gas nuclei with a bio- or hemocompatible solution. Denucleation serves to prevent the formation of blood-gas interfaces by largely or totally displacing the undissolved gas from the material. This prevents contact between the blood and the gas and thereby reduces or eliminates the blood-gas interface. Some priming techniques which have been previously employed to exclude air from a material prior to use include vacuum and carbon dioxide exposure, as well as saline, degassed saline, ethanol and acetone soaking. See generally, Osada et al., Am. J. Physiol.: Heart Circ Physiol. 234:H646-652 (1978); Kolobow and Tomlinson, J. Biomed. Mater. Res. Vol. 11, pp. 471-481(1977); Trudell et al., Trans. Am. Soc. Artif. Intern Organs 24A:320 (1978); and Madras et al., Trans. Am. Soc. Artif. Intern. Organs, 26: 153-157 (1980)).
Due to the porosity and hydrophobicity of many of the synthetic prostheses currently in use (e.g., PTFE), it was not surprising to find that the standard priming techniques, namely saline and degassed saline priming, were for all practical purposes completely ineffective at removing the gas nuclei from these biomaterials (Madras et al.). Vacuum, ethanol and acetone priming were shown to be more effective than saline or degassed saline priming, but these techniques and solutions were generally cumbersome, time consuming and/or resulted in only partial denucleation. For example, Ward et al. (Trans. Am. Soc. Art. Int. Org. 20:77-84 (1974)) and Ward and Forest (Ann. Biomed. Eng. 4:184-207 (1976)) demonstrated that vacuum priming of a silicone tube in a buffer solution at an absolute pressure of 1 torr for 20 hours was sufficient to remove enough gas nuclei to reduce platelet adhesion by 90%. This technique was ineffective, however, at removing gas nuclei which were trapped in crevices smaller than 2 microns.
Unlike vacuum priming, acetone priming has been shown to be quite efficient at removing even small gas nuclei from hydrophobic materials, but a subsequent soaking and flushing period is required during which time the acetone itself must be displaced from the denucleated material and replaced with saline, water, or other biocompatible liquids.
Priming of biomaterial, in this case PTFE, with ethanol has been shown to be significantly less effective at eliminating trapped gas than priming with acetone. Denucleation of PTFE with ethanol was incomplete even after 7 days of immersion. A 20 minute low pressure ethanol soak, while only partially effective in denucleating PTFE, was nonetheless found to improve the "effective porosity" of the material and to permit better tissue ingrowth. See, Trudell et al., and Madras et al.
In addition to increasing patency by employing denucleation techniques to reduce the formation of prosthesis associated-thrombi caused by undissolved gas nuclei, other methods for increasing patency have been attempted as well. For example, some investigators have tried to mimic the natural endothelium anti-thrombogenicity which is achieved at least partially in vivo due to its secretion of prostacyclin, a powerful antiplatelet agent. Processes which have been applied and tested both in vitro and in vivo with some success include the binding of anticlotting factors and antithrombogenics such as heparin, prostaglandins, serum albumin, and streptokinase in layers to the surface of prostheses. See e.g., Nichols et al., Circulation 70:843-850 (1984); Bennegard et al., Acta Anaesth. Scand. 26:112-120 (1982); Lin et al., Trans. Am. Soc. Artif. Intern. Organs 31:468-473 (1985); McRea et al., Trans. Am. Soc. Artif. Intern. Organs 27:511-516 (1981); Noishiki et al., Trans. Am. Soc. Artif. Intern. Organs 27:213-218 (1981); McRea and Kim, Trans. Am. Soc. Artif. Intern. Organs 24:746-752 (1978); Hoffman et al., Trans. Am. Soc. Artif. Intern. Organs 18:10-17 (1972); and Mori et al., Trans. Am. Soc. Artif. Intern. Organs 24:736-745 (1978).
In addition to antithrombotic therapeutics, agents bearing other therapeutic qualities have also been used in conjunction with synthetic biomaterials in order to reduce or alleviate disorders causally related to thrombi formation, as well as those generally associated with the use of synthetic prostheses. For instance, the risk of infection always exists when artificial substances are introduced into the body, and occurs preferentially at thrombotic sites. The likelihood of infection has been significantly reduced, however, when anti-viral and/or anti-bacterial therapeutics (collectively known as antibiotics) have been incorporated onto the surface or into the matrices of biomaterials used as sutures, catheters, and vascular prostheses. Greco and Harvey, J. Surg. Res. 36:237-243 (1984); Trooskin et al., Surgery 97:547-551 (1985); and Eddy et al., Plas. Reconstr. Surg. 78:504-512 (1986). Previous techniques stored such therapeutics in a layer bound to the surface of the biomaterial. These agents then diffused or were released from the bonding media over a period of hours or days.